In order to prevent a malfunction of a signal input unit for differential signals as used in a memory interface and the like, a permissible variation amount is defined for a voltage at which voltages of a positive signal and a negative signal of a pair of differential signals cross with each other (hereinafter referred to as “crossing voltage”).
Even when the crossing voltage falls within the defined permissible range at a signal output unit, the slew rate or the phase may differ between the positive signal and the negative signal due to inter-wiring crosstalk generated on a printed wiring board or a wiring length difference between a positive signal line and a negative signal line. The different slew rate or phase may vary the crossing voltage, thus failing to satisfy the defined permissible range of the crossing voltage at the signal input unit. The variation in the crossing voltage generated on the printed wiring board therefore needs to be reduced as much as possible.
In view of the foregoing, in PTL 1, it is described that a ground line is arranged between two signal lines that may generate inter-wiring crosstalk. Particularly when differential signal lines formed of a pair of signal lines are arranged adjacent to other differential signal lines formed of another pair of signal lines, two signal lines of different differential signal line pairs are arranged adjacent to each other. Accordingly, in PTL 1, there is disclosed a flexible wiring board in which a ground line is arranged between one differential signal line pair and another differential signal line pair, to thereby prevent crosstalk therebetween.
The use of a high-density mounting component such as a ball grid array (BGA), which is frequently used in recent years, requires a great number of signal lines to be led out from component pads. Correspondingly, the number of lead-out differential signal lines is increasing, and the amount of crosstalk between the differential signal lines is also increasing. In the printed wiring board disclosed in PTL 1, however, because a ground line is arranged between two signal lines that may generate inter-wiring crosstalk, a great number of ground lines are required to be arranged, with the result that a wiring region is enlarged to hinder high densification and miniaturization of the printed wiring board. The degree of freedom in design of the printed wiring board is also reduced. In addition, because the slew rate and the wiring density are increasing due to high-speed signal propagation, the crosstalk voltage generated between differential signal lines across a ground line is increasing. Thus, in order to obtain a sufficient effect of reducing the variation amount of the crossing voltage, the width of a ground line arranged between two signal lines needs to be further increased.
It is therefore an object of the present invention to reduce a variation amount of a crossing voltage of a pair of differential signals input to a signal input unit.